Electroacoustic transducer



oct- 23, 1962 R. K. DUNCAN 3,060,282

ELECTROACOUSTIC TRANSDUCER Filed July 26, 1957 2 Sheets-Sheet 1 4'0 CUEVS 3 RELA TIVE' DB N (A Q Q Q F/PEQUA'IVCY m crcz. ES PE? sscam;

Oct. 23, 1962 R. K. DUNCAN ELECTROACOUSTIC TRANSDUCER 2 Sheets-Sheet 2II\ II II II II III! IIII IIIIII II IIII II I! I\ II I I Filed July 26,1957 L H N R E T X E IIANETIC Fl EL D United States Patent Filed July26, 11957, Ser. No. 674,351 Claims. (Cl. 179-115) The present inventionrelates to electromagnetic acoustical transducers, and more particularlyto an improved magnetic diaphragm therefor.

Electromagnetic acoustical transducers generally employ a vibratilediaphragm of soft magnetic iron, which may 'be of the order of a fewthousandths of an inch thick. The diaphragm is usually supported orclamped about its periphery so as to be spaced in proper relation tomagnetic flux carrying members. Changes in the space relation of thediaphragm to these members changes the distribution of the magnetic fluxcarried in the magnetic circuit. Thus, in a reproducer, a change ofmagnetic flux in the magnetic structure will produce a movement of thediaphragm, and in a microphone the sound waves impinging on thediaphragm cause it to move, thereby changing the magnetic flux densityand generating an electromotive force in an electric coil.

In the construction of electromagnetic acoustical transducers, a numberof problems are encountered. Because the vibratile diaphragm has a lowvalue of mechanical resistance, it vibrates and resonates as arelatively high Q device. This produces an excessively high electricaloutput of a microphone at or near the resonant frequency of thediaphragm. Attempts to counteract this effect by acoustic dampingmaterial interposed between the diaphragm and the source of sound willlower the Q of the device, but this relationship varies with the ambientair pressure. Thus, where a microphone is to be used at high altitudesacoustic damping becomes ineffective. Stable mechanical damping, otherthan the dash pot type, is difiicult to obtain.

Another difiiculty encountered in the construction of electromagneticacoustical transducers of this type is encountered in the support of thediaphragm at its peripheral edge. Proper support requires that thediaphragm be clamped evenly around its periphery to avoid undue stressesand strains which might cause the diaphragm to vibrate with spuriousresonances and undesired effects. While a precision mechanical fit inthe supporting surfaces around the periphery of the diaphragm may servethe purpose, the expense involved is usually prohibitive for anyquantity production. Attempts to employ gaskets have been unsatisfactorybecause of changes due to aging and temperature variations.

In the construction of transducers of this type, it is quiteadvantageous to magnetize the structure after complete assembly. Suchoperation requires saturating the magnetic structure with magnetic fluxto obtain the desired magnetization of the permanent magnet. During thisoperation mechanical forces are exerted on the diaphragm which farexceed any forces apt to be encountered during normal operation of thedevice. Not infrequently in such magnetization operation the diaphragmis displaced to such an extent that it touches and adheres to the poleor pole pieces of the device and subsequently is practicallyinoperative. Any attempt to remedy this condition usually results in achange in the normal stillness of the diaphragm so that its restoringforce is not sufficient to prevent subsequent sticking to the polepieces when extremely high intensity sound is encountered.

In transducers of thistype, it is also necessary to protect the softiron diaphragm from oxidation or rusting. At extremely high soundlevels, the diaphragm is apt to 3,%d,22 Patented Get. as, 1962 strikethe pole piece which produces a high level noise which is detrimental tointelligent sound transmission.

In accordance with the present invention, the foregoing difficultiesencountered are obviated by an improved magnetic diaphragm which iscovered on both sides with a polyester film of exceptional physicalcharacteristics. This film protects the diaphragm against oxidation andrusting and reduces the detrimental noise effects encountered where highsound levels cause the diaphragm to strike the pole pieces. The filmcovering on the diaphragm also produces a desired damping so as tochange the resonant frequency of the diaphragm and minimize themagnitude thereof. During the magnetization operation the polyesterresinous film prevents the diaphragm from sticking to the pole pieces.At the same time, the film covering the diaphragm on both sides acts ina manner similar to a gasket in the peripheral clamping surfaces so thatthe diaphragm is more evenly supported without introducing unusualstresses and strains into the diaphragm.

It, therefore, is an object of the present invention to provide in anelectromagnetic acoustical transducer an arrangement which will minimizethe eflfects of the natural resonance of the diaphragm irrespective ofambient atmospheric pressure.

Another object of the invention is to provide for the even supporting ofthe peripheral portion of a diaphragm.

Still another object of the invention is to provide a transducer withmeans to prevent a transducer diaphragm from sticking to the magneticpole pieces while the structure is magnetized and also when in use inencountering high level sound intensities.

A further object of the invention is to simultaneously prevent oxidationand rusting of a ferrous diaphragm.

A still further object of the invention is .to provide a means forminimizing the effect of the diaphragm striking magnetic pole pieces.thereby to reduce the noise generated.

A still further object of the invention is to provide an improvedtransducer structure for minimizing the effects of stray magnetic fieldson the transducer.

Other and further objects of the invention subsequently will becomeapparent by reference to the following description taken in conjunctionwith the accompanying drawings, wherein:

FIGURES 1 and 2 are diagrammatic representations of electromagneticacoustical transducers, to which the present invention has been applied;

FIGURE 3 is an exploded view of a transducer incorporating the featuresof the present invention;

FIGURE 4 is a partial cross-sectional view through a modified type ofdiaphragm constructed in accordance with the present invention;

FIGURE 5 is a diagrammatic representation of the coil structure employedin transducers of the types shown to minimize the effects of externalmagnetic fields; and

FIGURE 6 is a curve showing the improved response obtained by theapplication of the present invention.

Referring to FIGURE 1 there is diagrammatically shown a transducer whichwill serve as a microphone. This comprises a housing having two similarend pieces 11 each provided with a plurality of apertures 12 so thatsound may enter into the cavities within the structure. Similarcylindrical housing members 13 are interposed between the ends 11 and aferrous diaphragm 14. Within the two chambers formed by the diaphragm 14are two similar magnetic structures each comprising a permanent magnet15 surrounded by a winding or coil 16.

Where a diaphragm with the housing forms a single cavity, the structureis similar to that shown in FIGURE 2. Therein a non-magnetic housing 17supports a diaphragm 18 retained in position by a suitable clamping ring19. Within the housing 17 is a permanent magnet 21 having pole pieceextensions 22 surrounded by windings 23. In accordance with the presentinvention, the diaphragms 14 and 18 of FIGURES l and 2 preferably iscoated on both sides with a polyester resinous film, such as Du PontMylar.

A preferred transducer particularly suited for high altitude microphoneservice is shown in the exploded view of FIGURE 3. This structurecomprises a cylindrical casing 24 having therein two spacing rings 25which are arranged to engage the clamp the peripheral edge portions of adiaphragm 26. The diaphragm 26 has a central ferrous sheet metal portion27 coated on opposite sides by a polyester resinous film 28 whichpreferably is baked on the sheet metal portion 27. The spacing rings 25are of soft iron and serve as a part of a magnetic circuit which iscompleted by an end piece 29 having suitable sound apertures 31 therein.Each end piece 29 abuts a permanent magnet member 32 which is surroundedby an electromagnetic coil 33. The end piece 29 is engaged by a threadedclamping ring 34, to which is applied a flexible membrane covering 35 toexclude moisture from the structure. In order that the chambers formedwithin the housing 24 by the diaphragm 26 may be suitably vented toatmospheric pressure, the rings 25 are each provided with openings 36which align with openings 37 in the housing 24, which in turn arealigned with openings 38 in a conduit 39. The conduit 39 may pass in theinstance of a facial mask to the exterior of the mask to vent thestructure to atmospheric pressure.

It has been mentioned that the diaphragm 26, which is of the order of0.005 inch thick, is covered with a film of polyester resin which may beof the order of .0025 inch thick. The normal gap between the magneticmember 32 and the diaphragm 26 is of the order of .007 inch. The normalamplitude of motion of the diaphragm is approximately .003 inch with amaximum travel of the diaphragm in one direction of .0045 inch.

Where it is desired to increase still further the stiffness of thediaphragm, a modified diaphragm structure may be employed as illustratedin the partial cross-sectional view of FIGURE 4. Therein two thinnersheets of ferrous material 41 are cemented to a film 42 of polyestermaterial such as the Du Pont Mylar. The outer surfaces of the twocemented together ferrous diaphragm members 41 are covered by Mylarfilms 43. While the polyester film produces the damping effect, therealso may be some loss in the response sensitivity, but this is believedto be offset by some of the other features gained by the use of thepolyester film. The film operates in a manner similar to a gasket at theclamping surfaces to distribute the clamping pressure evenly and thusavoid an expensive construction and eliminate undue detrimental strainsin the metallic diaphragm. The elimination of such strains, therefore,insures an improved response characteristic.

In the winding of the coils 33 of FIGURE 3, the coils are wound in themanner diagrammatically shown in FIGURE 5. It will be noted that themagnetic structures 32 are so magnetized that the north poles are in thesame direction relative to the south poles of the structures. Theconnections of the coils 33 are such that a displacement of thediaphragm 26 from the solid line to the dash-dot line representationwill generate in the coils 33 a potential tending to cause a flow ofcurrent as indicated by the arrows. Those skilled in the art willreadily appreciate that an external magnetic field, such as thatrepresented by the dotted line arrows, will generate voltages in thecoils 33 which are in opposition to each other, and hence no detrimentalvoltages or cur rents will appear in the circuit to which the transduceris connected due to the external magnetic field.

The improvement resulting from the application of the polyester film toopposite sides of the ferrous metal dia- 4. phragm is illustrated in thecomparison of the curves shown in FIGURE 6 wherein curve A is theresponse characteristic of a typical diaphragm which does not have thepolyester film, whereas curve B is the same diaphragm coated with a filmin accordance with the teaching of the present invention.

Accordingly, the present invention overcomes many of the detrimentaleffects and problems heretofore encountered in prior structures. Fromthe representations of the preferred embodiments as shown in FIGURES 3and 5, it becomes apparent that it is possible to magnetize thestructure after assembly and still prevent the diaphragm 26 fromremaining stuck to one of the magnetic pole pieces. This eliminates thedifficulty encountered in prior constructions where a subsequent forcingof the diaphragm from the pole piece produced a change in the magneticfield and often resulted in a change in the normal stiffness of thediaphragm. Without magnetic saturation between the magnetic members inan ideal magnetic circuit, the magnetic force of attraction of adiaphragm is proportional to the square of the flux density. The fluxdensity is approximately proportional to the length of the air gapbetween the magnetic members for operation over a restricted portion ofthe demagnetization curve. In a typical practical magnetic circuit theresulting force of attraction, while far from being a perfect square lawfunction with respect to separation, is greater than a linear relation.The restoring force of the diaphragm, due to its mechanical stiffness,is approximately proportional to the magnitude of the displacement ofthe diaphragm. It thus may be seen that a condition can exist such thatas the diaphragm is displaced up to a certain point, it will return toits normal position when the displacing force is removed. If thediaphragm should pass a critical point, the diaphragm will continue tomove toward the pole piece until it rests upon it. The application ofthe film is such, however, that this latter condition is obviated.

While for the purpose of illustrating and describing the inventionparticular embodiments have been shown in the drawings, it is to beunderstood that the invention is not to be limited thereby since suchvariations and modifications are contemplated as may be commensuratewith the spirit and scope of the appended claims.

I claim as my invention:

1. An electromagnetic acoustical transducer comprising a hollowcylindrical housing, a circular diaphragm of ferromagnetic materialdisposed within the housing and provided with a coating of polyesterresin on the surfaces thereof, said diaphragm having a diameterapproximately equal to the inner diameter of the housing, a cylindricalsleeve of ferromagnetic material disposed within the housing on eachside of the diaphragm and abutting the diaphragm, an end plate offerromagnetic material having a plurality of apertures disposed abuttingthe side of each of the sleeves opposite the diaphragm, a magnet mountedon each of the end plates and extending toward the diaphragm, and a coildisposed about each of the magnets.

'2. An electromagnetic acoustical transducer comprising the elements ofclaim 1 wherein the magnets have cylindrical shapes and the central axesof the magnets are disposed upon a common axis normal to the diaphragm,and the coils disposed about the magnet are provided with the samenumber of turns and connected in series opposition.

3. An electromagnetic acoustical transducer comprising the elements ofclaim 1 wherein the diaphragm comprises two spaced sheets offerromagnetic material, a sheet of polyester resin disposed between andabutting the sheets of ferromagnetic material, and a coating ofpolyester resin disposed on the surfaces of the ferromagnetic sheetsopposite the sheet of polyester resin.

4. An electromagnetic acoustical transducer comprising a hollowcylindrical housing having threads extending therein on the innersurface thereof from each end, a circular diaphragm of ferromagneticmaterial disposed within the housing and provided with a coating ofpolyester resin on the surfaces thereof, said diaphragm having adiameter approximately equal the inner diameter of the housing, acylindrical sleeve of ferromagnetic material disposed within the housingon each side of the diaphragm and abutting the diaphragm, an end plateof ferromagnetic material having a plurality of apertures disposedabutting the side of each of the sleeves opposite the diaphragm, amagnet mounted to each of the end plates and extending toward thediaphragm, a coil disposed about each magnet, a threaded ring engagingthe threads on each end of the housing and securing the end platesbetween the ring and the confronting sleeve, and a moisture barriersealed to the side of the ring opposite the end plates.

5. An electromagnetic acoustical transducer comprising the elements ofclaim 4 wherein the diaphragm comprises two spaced sheets offerromagnetic material, a sheet of polyester resin disposed between andabutting the sheets of ferromagnetic material, and a coating ofpolyester resin disposed on the surfaces of the ferromagnetic sheetsopposite the sheet of polyester resin.

References Cited in the file of this patent UNITED STATES PATENTS1,065,888 Sabine June 24, 1,362,008 Kane Dec. 14, 1,630,380 Hanna May31, 1,660,864 Conrad Feb. 28, 1,822,095 High Sept. 8, 1,945,986 RichmondFeb. 16, 1,944,323 Kelching Jan. 23, 2,044,608 Harford June 16,2,166,326 Reisz July 18, 2,274,513 Wells Feb. 24, 2,360,796 'RobertonOct. 17, 2,535,757 Root Dec. 26, 2,545,007 Schultheis Mar. 13, 2,754,280Brown et al. July 10, 2,802,764 Slayter et al. Aug. 13, 2,892,747 DyeJune 30,

FOREIGN PATENTS 354,018 Great Britain Aug. 6, 289,422 Switzerland July1,

OTHER REFERENCES Fikendscher, Serial No. 397,138, May 11, 1943.

1. AN ELECTROMAGNETIC ACOUSTICAL TRANSDUCER COMPRISING A HOLLOWCYLINDRICAL HOUSING, A CIRCULAR DIAPHRAGM OF FERROMAGNETIC MATERIALDISPOSED WITHIN THE HOUSING AND PROVIDED WITH A COATING OF POLYESTERRESIN ON THE SURFACES THEREOF, SAID DIAPHRAGM HAVING A DIAMETERAPPROXIMATELY EQUAL TO THE INNER DIAMETER OF THE HOUSING, A CYLINDRICALSLEEVE OF FERROMAGNETIC MATERIAL DISPOSED WITHIN THE HOUSING ON EACHSIDE OF THE DIAPHRAGM AND ABUTTING THE DIAPHRAGM, AN END PLATE OFFERROMAGNETIC MATERIAL HAVING A PLURALITY OF APERTURES DISPOSED ABUTTINGTHE SIDE OF EACH OF THE SLEEVES OPPOSITE THE DIAPHRAGM, A MAGNET MOUNTEDON EACH OF THE END PLATES AND EXTENDING TOWARD THE DIPHRAGM, AND A COILDISPOSED ABOUT EACH OF THE MAGNETS.